1. Field of the Invention
This invention relates generally to a system and method for controlling the flow of an anode gas to a fuel cell stack in response to an injector that has failed in a stuck open position and, more particularly, to a system and method for controlling the flow of an anode gas to a fuel cell stack in response to an injector that has failed in a stuck open position, where the system uses a pressure regulator that is controlled to an anode sub-system pressure to control the flow of the anode gas.
2. Discussion of the Related Art
Hydrogen is a very attractive fuel because it is clean and can be used to efficiently produce electricity in a fuel cell. The automotive industry expends significant resources in the development of hydrogen fuel cell systems as a source of power for vehicles. Such vehicles would be more efficient and generate fewer emissions than today's vehicles employing internal combustion engines. Fuel cell vehicles are expected to rapidly increase in popularity in the near future in the automotive marketplace.
Proton exchange membrane fuel cells (PEMFC) are a popular fuel cell for vehicles. The PEMFC generally includes a solid polymer electrolyte proton conducting membrane, such as a perfluorosulfonic acid membrane. The anode and cathode typically, but not always, include finely divided catalytic particles, usually a highly active catalyst such as platinum (Pt) that is typically supported on carbon particles and mixed with an ionomer. The catalytic mixture is deposited on opposing sides of the membrane. The combination of the anode catalytic mixture, the cathode catalytic mixture and the membrane define a membrane electrode assembly (MEA). MEAs are relatively expensive to manufacture and require certain conditions for effective operation.
Several fuel cells are typically combined in a fuel cell stack to generate the desired power. For example, a typical fuel cell stack for a vehicle may have two hundred or more stacked fuel cells. The fuel cell stack receives a cathode input gas, typically a flow of air forced through the stack by a compressor. Not all of the oxygen is consumed by the stack and some of the air is output as a cathode exhaust gas that may include water as a stack by-product. The fuel cell stack also receives an anode hydrogen input gas that flows into the anode side of the stack.
A fuel cell stack includes a series of bipolar plates positioned between the several MEAs in the stack, where the bipolar plates and the MEAs are positioned between two end plates. The bipolar plates include an anode side and a cathode side for adjacent fuel cells in the stack. Anode gas flow fields are provided on the anode side of the bipolar plates that allow the anode reactant gas to flow to the respective MEA. Cathode gas flow fields are provided on the cathode side of the bipolar plates that allow the cathode reactant gas to flow to the respective MEA. One end plate includes anode gas flow channels, and the other end plate includes cathode gas flow channels. The bipolar plates and end plates are made of a conductive material, such as stainless steel or a conductive composite. The end plates conduct the electricity generated by the fuel cells out of the stack. The bipolar plates also include flow channels through which a cooling fluid flows.
Typically, hydrogen gas for the fuel cell system is stored at high pressure in a tank system including one or more interconnected pressure vessels on the vehicle to provide the hydrogen gas fuel necessary for the fuel cell stack. The pressure within the vessels can be 700 bar or more. In one known design, the pressure vessels include an inner plastic liner that provides a gas tight seal for the hydrogen gas, and an outer carbon fiber composite layer that provides the structural integrity of the vessel.
A hydrogen gas storage system typically includes at least one pressure regulator as part of the various and numerous valves, gauges, and fittings necessary for operation of the hydrogen storage system, where the pressure regulator reduces the pressure of the hydrogen gas from the high pressure in the vessels to a constant pressure suitable for the fuel cell stack. Various pressure regulators are known in the art to provide this function, including mechanical pressure regulators and electronic pressure regulators.
Most fuel cell systems employ one or more injectors for injecting the reduced pressure hydrogen gas into the anode side of the fuel cell stack. The injectors are typically controlled by a pulse width modulation (PWM) signal having a certain duty cycle and frequency that provides the desired mass flow of the hydrogen gas for a commanded stack current density. In one known fuel cell system control, the duty cycle and frequency of the injector is set based on the pressure within an anode sub-system.
For example, the pressure regulator may reduce the pressure of the hydrogen gas from a tank pressure of up to 875 Mpa down to approximately 800 kpa to provide a constant supply pressure to the injector. The injector then provides a pulsed flow to regulate the stack anode pressure in a range between 100 and 300 kpa. In maintaining the anode pressure, the hydrogen flow needed to power the fuel cell system is satisfied. It is important to note that both the regulator and the injector are needed to maintain an accurate pressure control over the full range of power transients for vehicle operation. The injector frequency and pulse width are controlled by feedback from an anode pressure sensor. In addition, the injector when open, may provide a high velocity flow to an ejector that recycles gas flow from the stack outlet to the stack inlet. This pulsed operation in conjunction with the recycled flow is crucial to maintain durable and stable system operation.
Over the life of a vehicle, an injector will undergo hundreds of millions of cycles of operation. During this time, there is a potential for the injector to stick open, which can cause an uncontrolled anode pressure rise. The pressure rise must be detected and mitigated before the burst pressure of the stack or other system components is reached, which can damage the system and release hydrogen gas to the environment. An anode pressure sensor is provided downstream of the injector, where if the sensor detects a rise in the anode pressure, the system identifies a failed injector in the open position. Opening the anode valves is not used for this mitigation because the valve flow cannot match the injector flow and also because the hydrogen flow through the valve can lead to unsafe exhaust emissions. As a result, the typically failure strategy has been to shut the system down once a maximum anode pressure is exceeded. However, this may strand the vehicle driver and potentially put the driver in an unsafe traffic situation, which is obviously an undesirable condition depending on where the vehicle is at a particular point in time.